1. Field of the Invention
The present invention relates to electrophotography and, more particularly, to a light emitting diode (LED) array print head applicable to a printer which uses an electrophotographic procedure.
2. Description of the Prior Art
A family of printers of the type adopting the principle of electrophotography includes a laser beam printer which uses a laser beam and an LED printer which uses an LED array, as well known in the art. All the printers of this type are equipped with an optical print head which serves as a one-dimensional or linear scanning optical writing device for producing an electrostatic latent image on the surface of a photoconductive drum. The optical print head using a laser beam and the optical head using an LED array as stated above are generally referred to as a laser beam scanner and an LED array print head, respectively.
A laser beam scanner scans a photoconductive element one-dimensionally at a high speed by a laser beam which is reflected by a mirror which is in turn rotated by a motor. Such a scanner, therefore, needs a substantial period of time to reach a constant speed after the start of operation, resulting in a construction which is of disproportionately large size. In contrast, an LED array print head which is implemented with a one-dimensional array of LEDs is capable of electronically scanning a photoconductive element with no movable members included therein. For this reason, the LED array print head substantially eliminates the need for the buildup time after the start of operation which is particular to the laser beam scanner as stated above, and in addition it can be constructed to be of a small size.
In an LED array print head, LEDs arranged in an array are selectively energized in response to a data signal which is fed to the print head from a processing system or like external equipment. Light issuing from particular ones of the LEDs is focused by a rod lens array onto the surface of a photoconductive element, so that an image represented by the light, such as an electrostatic latent image, is formed on the photoconductive element.
An LED array print head includes a plurality of LED array devices each being constituted by a linear array of numerous LEDs. The LED array devices are individually fixed to, for example, a ceramic substrate via a common electrode for LEDs which is provided on a surface opposite to the surface where light emitting portions of the LEDs are positioned. Independent electrodes of each device which are associated one-to-one with the LEDs are connected by bonding wires to conductive leads which are provided on the substrate by printing technology. Also mounted on the substrate are drivers which are adapted to drive the LEDs and each of which is constructed into an integrated circuit, connecting terminals of the drivers being also connected to printed conductive leads by bonding wires.
An LED array print head has a number of LEDs each of which is associated with a respective one of a number of pixels which are sequentially defined in a main or horizontal scanning direction of an image to be printed out. This gives rise to certain problems, however. Specifically, since such a number of LEDs have to be accompanied by a corresponding number of independent electrodes and therefore by a corresponding number of bonding wires, a prohibitive number of assembling steps are required, thus resulting in an expensive print head. Another problem is derived from the fact that while the common electrode associated with the LEDs is provided on the opposite side to the light emitting side of the LEDs, the independent electrodes each being associated with a respective one of the LEDs are located on the same side as the light emitting side of the LEDs. In this condition, assuming that the independent electrodes of the LED array device are connected to conductive leads of an optical fiber plate, then extra means such as bonding wires have to be used to connect the LEDs. This not only aggravates the difficulty of assembly but also adds to the cost.
A method of mounting an LED array without relying on bonding wire which are disadvantageous for the reasons discussed above is disclosed in Japanese Patent Publication No. 8074/1984. The method disclosed in this Patent Publication consists in mounted an LED array on an optical fiber plate in such an orientation that the light emitting suface of the LED array faces the optical fiber plate. However, when an optical fiber plate is used as an imaging optical element, the allowable distance between the optical fiber plate and a photoconductive drum is severely limited so that toner particles may be deposited on the plate.